Sedimentary rocks give us important information about what the world was like millions of years ago, such as the location of the
source, or
provenance area, from which the sediment originated, the kinds of source rocks, and the
paleocurrents (the direction of flow that deposited the sedimentary grains and how the direction changed with time).
Rock types and structures allow the geologist to determine if the sediments were deposited by glaciers, rivers, lakes, deltas, beaches, sand dunes, wind, lagoons, continental shelf currents, reefs, or deeper ocean waters. High‐energy environments such as steep river channels usually deposit coarse arkosic sandstones or conglomerates. Beaches and barrier islands consist of well‐rounded quartz sandstone. Lower‐energy environments like lake beds, deltas, lagoons, and the deep ocean can be identified by the finer‐grained rocks such as shale and siltstone. Limestones usually identify marine reef environments.
An integration of this information over a large region leads to the reconstruction of the depositional environment—what the region was like in the geologic past. This three‐dimensional reconstruction, over what can be thousands of square kilometers, can be detailed enough to identify such events as flooding and fluctuations in sea level that happened hundreds of millions of years ago.